1. Field of the Invention
The present invention relates to a single crystal pulling method and a single crystal holding device which is mounted in a single crystal fabricating apparatus using the Czochralski process (CZ process) and preferably used to fabricate a single crystal having a large weight.
2. Description of the Prior Art
The single crystal is generally fabricated using the Czochralski process. The CZ process is performed as follows. A quartz crucible, which is installed in a single crystal fabricating device, is filled with a polycrystalline silicon (poly-Si). The poly-Si is heated and made molten by a heater provided around the quartz crucible. Seed crystal, which is attached to a seed holder, is immersed in the melt. While the seed holder and quartz crucible are rotated in the same direction as or an opposite direction to each other, the seed holder is pulled up so that the single crystal silicon is grown to have a predetermined diameter and length.
In the seed crystal, dislocation occurs owing to thermal shock when it is immersed in the melt. In order to remove the dislocation, a neck having a diameter of 3-4 mm is formed in the lower portion of the seed crystal using the dash neck technique so that the dislocation is diverted toward the surface of the neck portion. After the removal of the dislocation is confirmed, a shoulder is formed to extend the single crystal to have a prescribed diameter. Thereafter, a straight waist to be single crystal product (hereafter a straight waist product portion) will be formed.
Meanwhile, in recent years, because of an increase in the weight of the single crystal due to an increase of its diameter and length, the strength of the neck is approaching to the limit. Therefore, the conventional pulling method may break the neck and cannot grow the single crystal safely. To overcome such an inconvenience, a device and method have been proposed which replace a part of the weight load from the neck into a holding tool while the single crystal is grown. Such a device or method, which supports most of the weight of the single crystal by the holding tool, prevents the neck from being broken and the single crystal from dropping from the holding tool even when the neck is broken.
However, the conventional single crystal pulling method or holding device has several problems.
(1) It is known that the single crystal of silicon is deformed plastically within a temperature range from the melting point to 75.degree. C. Therefore, when the silicon single crystal is held in the above temperature range, it is deformed plastically. This makes it impossible to hold the single crystal safely and surely, and may generate dislocation in the single crystal. In addition, contact of a holding tool with the single crystal in the aforementioned high temperature range will contaminate the single crystal.
(2) In Japanese Patent Publication.(JP-B) No. 5-65477, several methods are disclosed for forming a constriction of the single crystal used when the single crystal is held for using the holding tool in the step of increasing the diameter after the neck has been formed. The constriction can be formed by controlling the shape of the single crystal in such a manner that the crystal pulling speed and melt are adjusted. However, it is difficult to form the desired shape with good reproducibility so that the relative positions of the holding tool and constriction differ for different single crystals. This makes it impossible to hold the single crystal safely and surely.
(3) JP-A (Laid-Open) 9-2893 discloses a method of holding the single crystal without forming the constriction in which a hanging tool is engaged in the concave portion in a bulge generated after the dislocation is removed in the neck. This method, however, fails to refer to the mechanical sandwiching force applied to the concave portion. Since the concave portion created after the dislocation has been removed in the neck has a small diameter, it is necessary to control the holding force in order to hold the single crystal safely and surely.
(4) In the method of (3), if the pinching force applied to the concave portion having a small diameter is balanced completely, no problem occurs. But if the center axis thereof is shifted, the picking force will on the concave portion as a bending force. This may break the concave portion easily. In this case, there are dangers of damaging the pulling furnace body or accidents resulting in injury and death. The resulting damage is immeasurable.
(5) In the method of (3), in the case of a method of holding an inverted right conical portion at the straight waist or concave portion of the bulge created intentionally, a hanging tool made of a different substance from the single crystal is brought into direct contact with the area to be used as a product so that it will be contaminated.
(6) In order to replace the weight of the crystal from the neck into the holding tool smoothly, it is desired that the weight of the crystal is small at the time of replacement. If the weight of the crystal is small, dislocation, even if it occurs, can be dealt with by the conventional method. The crystal free from the dislocation can be obtained by the subsequent pulling step. However, the straight waist of the single crystal can be actually picked up only after the single crystal has been grown to a prescribed length. The first reason is that short crystal is high in temperature so that plastic deformation, dislocation or contamination is likely to occur.
The second reason is attributable to the structure of the crystal pulling device. Specifically, a contracted bellows, a gate chamber and a top chamber are located beneath the holding tool so that the crystal length from a surface of the melt to the position of holding is lengthened necessarily. If the arm of the holding tool is lengthened correspondingly, it will be opened and useless when a picking force is applied. Accordingly, the holding operation can be substantially carried out only after the single crystal has been grown to a considerable length, i.e. the weight of crystal has been increased.